1. Field of the Invention
The present invention relates to a track ball device and, more particularly, to an electrostatic sensing scheme and circuit for directly sensing orientation of a track ball.
2. Description of the Related Art
A track ball, like a mouse or joy stick, controls the position of a cursor on a computer screen. The track ball unit stays in one place, and the ball is manually rotated for positioning the cursor.
A typical track ball unit is described in U.S. Pat. No. 5,248,961 and consists of a ball rotatably arranged in a case. A portion of the ball protrudes from an opening in the top of the case. An x-axis driven roller and a y-axis driven roller are arranged in the case at right angles to each other and are frictionally coupled with the ball. Rotary encoders are affixed to the shafts of the rollers. As the ball is rotated, the x-axis and y-axis rollers and corresponding encoders are driven in accordance with the components of rotation in the x and y directions. Typically, the rotary encoders are optical encoders including a light shielding plate and light emitting and receiving elements. The light shielding plate consists of a plate having a predetermined pattern of slits and is mounted on the shaft of each of the x-axis and y-axis driven rollers. A light emitting element and a light receiving element are disposed opposite each other across each of the light shielding plates. Alternative arrangements include the use of magnetic type encoders or mechanical type encoders in place of the optical encoders.
One disadvantage of these typical track ball units is that they require many mechanical parts including moving parts, flexing parts, friction rollers, etc. Accordingly, there are many parts that tend to foul, wear out or break.
Sun Microsystems Computer Corp. manufactures a direct-sensing optical mouse with no moving parts except for the mouse body itself. For example, see U.S. Pat. Nos. 4,364,035 and 4,546,347. The mouse body rides over a reflective pad with a cross-hatch pattern printed on it. A disadvantage of this system is that it fails when the mouse body is turned askew to the cross-hatch pattern or the reflective pads gets scratched or dirty.
In addition, the power requirements for commercially available optical sources and detectors limits the use of optical sensors in a battery operated track ball device.
U.S. Pat. No. 5,049,824 relates to a phase discrimination type electrostatic capacity detector for use as a displacement detector and a length measuring device. A first electrode plate has transmitting electrode elements and a receiving electrode oriented parallel to the transmitting electrodes. A second electrode plate has a plurality of coupling electrodes and a plurality of earth electrodes disposed alternately therewith. The first electrode plate is mounted on a stationary body of a dial gauge type length measuring instrument. The second electrode plate is mounted on a movable sliding member. By applying an electric signal to respective transmitting electrodes, the amount of displacement of the sliding member with respect to the stationary body can be detected. However, although such a device is useful for determining displacement in a linear one-dimensional direction, it is not particularly useful in a device that must measure displacement in a two dimensional plane or surface.
Accordingly, there is a need to overcome the above-noted problems associated with the prior art.